Electric drive systems on ships (e.g. on all-electric ships) typically comprise one or more electric drive motors, each of which is provided for driving one propulsion unit (e.g. a propeller), which are fed in each case by way of a converter from an electrical power network of the ship (often also referred to as a “propulsion network”). The electrical power network is in turn fed by one or more diesel generators. In this case the electrical power network has a voltage of predetermined fixed amplitude and frequency, e.g. a medium voltage having a rated voltage of 6.6 kV at a rated frequency of 60 Hz. A transformer may also be connected into the circuit between the converter and the electrical power network. The converters convert the (where necessary down-transformed) supply voltage into a voltage required for operation of the drive motors and having a different amplitude and frequency from the supply voltage.
Low-voltage power-consuming loads on board a ship (e.g. navigation and control equipment, public address system, lighting) are supplied by means of a separate electrical onboard power network that typically has a rated voltage of 400 V at a rated frequency of 50 Hz or 440V at 60 Hz. The electrical onboard power network can be fed with electrical energy by dedicated electrical onboard power network generators independently of the propulsion network. Alternatively the electrical onboard power network can be supplied from the propulsion network by way of an electrical onboard power network converter and where necessary a transformer. The electrical onboard power network converter, in combination with the transformer where necessary, converts the voltage of the propulsion network into a voltage having the amplitude and frequency of the electrical onboard power network.
A great advantage of this solution is that backlash effects on the propulsion network due to sudden load shocks (e.g. when a propeller lifts out of the water and is reimmersed in heavy seas) can be avoided by way of the converter if the latter is dimensioned with a commensurately large capacity. Apart from many other advantages, however, these drive concepts have the disadvantage that they require a comparatively large number of converters for converting the voltage in the propulsion network, with corresponding space requirements and costs.
A further known electric drive solution which manages without converters of such a type resides in coupling the generators and the drive motors to each other without intermediately connected converters. In a drive solution of such a type, one or more speed-variable drive motors without intermediately connected converters are operated directly with the voltage of variable amplitude and variable frequency that is generated by one or more speed-variable generators.
The open- and/or closed-loop control of the motors and consequently of the propulsion units is therefore accomplished indirectly by way of open- and/or closed-loop control of the internal combustion engines for driving the generators. In this case the drive motors are permanently electrically coupled to the generators, i.e. a rotational movement of the generators effects a correspondingly proportional rotational movement of the electric drive motors. The function of a mechanical shaft is therefore replicated with the aid of electric machines. A drive solution of this type is frequently referred to as an “electric shaft”.
It is also known here to couple out electrical energy from the electric shaft by way of an electrical onboard power network converter and where necessary a transformer, i.e. an electrical onboard power network converter, in combination with a transformer where necessary, converts the voltage of variable amplitude and variable frequency generated by the generator(s) into a voltage having constant amplitude and constant frequency for an electrical onboard power network.
Multihulled vessels such as e.g. catamarans or trimarans capable of speeds in excess of 40 knots are particularly suitable for express ferries and navy applications in which high speeds are important. They enjoy increasing popularity for that reason. For propulsion purposes these vessels have waterjets, for example, which are mechanically coupled directly to diesel engines or gas turbines and are driven by the latter. The mechanical direct drive does however lead to vessel design constraints which prevent the vessel being designed in an optimal manner (in respect, inter alia, of hydrodynamic and functionality aspects).